The invention is in the field of multichannel optical transmission devices, in particular for parallel multichannel data transmission by means of a corresponding plurality of optical waveguides, and relates to a transmission device having a transmission unit comprising a plurality of useful lasers arranged in a row, where the optically active zones of the useful lasers, when driven, emit useful radiation vertically with respect to the covering area of the transmission unit, and having a monitor unit for monitoring, which comprises at least one monitoring laser arranged on the row of useful lasers, the optically active zone of which monitoring laser, when driven, emits radiation vertically with respect to the same covering area of the transmission unit, which radiation at least partly passes to a radiation-sensitive area of a monitor receiver.
It is known (EP 0622874 A1) in connection with laser transmitters that the optical power radiated by the laser is subject in particular to temperature- and aging-dependent fluctuations. With regard to laser safety, moreover, it must be ensured that laser light emerging from an externally accessible optical interface (so-called optical port) remains below a power limit value. In the event of an impermissible increase in the laser power above said limit value, for example owing to a fault in the electronic drive circuit, the laser must be reliably switched off. For the purposes mentioned above, regulating devices are customary which readjust the driving of the laser in order to obtain a constant output power lying below permissible limit values. To that end, the optical power must be monitored. To that end, EP 0622874 A1 discloses regulation of the temperature- and aging-dependent optical power of an edge-emitting laser to whose rear radiation region a monitoring or monitor diode is coupled. The coupling is effected by means of a depression which is arranged in a common support and at which the radiation emitted at the rear passes via at least one reflecting surface onto the optically sensitive zone of the monitor diode. This known design is unsuitable, however, for monitoring and regulating laser devices which radiate vertically with respect to the covering area (so-called VCSEL).
The international patent application WO 97/25638 discloses a multichannel optical transmission device of the type mentioned in the introduction whose transmission unit is formed by a laser array containing a plurality of individually driveable useful lasers arranged in a row. The optically active zones of the useful lasers are designed in such a way that their useful radiation emerges vertically with respect to the covering area. The emitted radiation of the first semiconductor laser in the row is not, by contrast, used as useful radiation but rather serves for regulating the drive current of all the semiconductor lasers. This first single laser thus functions as a monitoring laser; the radiation thereof passes through a depression on that side of a base body which faces the laser array, by multiple reflection, onto the radiation-sensitive area of a monitor receiver arranged next to the laser array. The monitor receiver is arranged essentially on the same plane as the laser array.
This known transmission device allows reliable regulation of the laser output powers, but the multiple deflection of the radiation of the monitoring laser and the mounting can prove to be problematic. In cases where complete potting of the transmission device is desirable, there is the risk of air inclusions forming at the deflection mirrors, which air inclusions reduce the coupling efficiency. Overall, only a comparatively small proportion of the radiation emitted at the monitoring laser is available for regulation purposes.
The object of the invention consists, therefore, in developing a transmission device to the effect that a comparatively high coupling efficiency between the monitoring laser and the monitor receiver is ensured in conjunction with a design of the transmission device which is simple and facilitates production.
In the case of a transmission device of.the type mentioned in the introduction, this object is achieved according to the invention by virtue of the fact that the monitor receiver is arranged on a plane lying above the covering area of the transmission unit, and that the monitor receiver projects above the monitoring laser like a cantilever support in such a way that its radiation-sensitive area is opposite the optically active zone of the monitoring laser.
The monitoring laser should have to the greatest possible extent identical electro optical properties, in particular with regard to the temperature response and aging behavior, to those of the useful lasers. This can advantageously be achieved in a particularly simple manner by virtue of the fact that the monitoring laser is part of the transmission unit. In other words, a transmission unit (preferably a laser array) is used in which not all of the activatable individual lasers are used as useful lasers, rather at least one of the lasers serves as a monitoring laser.
A significant advantage of the invention is that it is possible to reliably regulate the transmission device with comparatively few components, which are simple to produce and mount. The monitor receiver arrangement may advantageously be mounted independently of the transmission unit and initially tested. A further considerable advantage of the transmission device according to the invention is that virtually hundred percent overcoupling of the monitoring laser power to the monitor receiver is possible. Highly effective regulation of the transmission device can be achieved as a result. An advantageous aspect of the invention is that the optically active zone of the monitoring laser has situated opposite it a radiation-sensitive area of the monitor receiver, which area is comparatively large in relation to said zone; this allows comparatively large positional tolerances with regard to the monitor mounting. Finally, the transmission device according to the invention manages without additional optical imaging or reflecting elements in the optical coupling between monitoring laser and monitor receiver, which contributes to a considerable reduction of the production outlay.
A refinement of the transmission device according to the invention which is preferred in terms of production engineering provides for electrical connections of the monitor receiver to lie on the same side as the radiation-sensitive area.
This makes it possible to mount and to make contact with the monitor receiver using so-called flip-chip technology, in which case it is advantageously possible to utilize self-centering effects by means of solder bumps previously applied to corresponding contact areas.
In order to protect the transmission device against ambient influences, an advantageous development of the invention provides for the space between the optically active zone of the monitoring laser and the radiation-sensitive area of the monitor receiver to be filled with a plastic which is transmissive to the emitted radiation.
A further reduction of the individual parts required for the transmission device according to the invention can be achieved, according to an advantageous development of the invention, by virtue of the fact that the transmission unit is arranged on a lower mounting plane and the monitor receiver is arranged on a laterally superior mounting plane of a common support. The support has mounting areas at different heights, contact being made with the monitor receiver on the mounting area at the higher level preferably using flip-chip technology, with utilization of the self-alignment effect in the course of solder bump mounting. Particularly reliable regulation using balanced regulating input signals can be achieved, according to a preferred refinement of the invention, by virtue of the fact that adjoining the row of useful lasers there is arranged in each case a monitor unit with a monitoring laser and a monitor receiver whose radiation-sensitive area faces the optically active zone of the respective monitoring laser.
Exemplary embodiments of a transmission device according to the invention are explained below with reference to the drawings.